The secular approximation is usually valid in a quantum optics setting, where the energy level spacings are sufficiently large, but fails when there are nearly degenerate energy levels. Recently, many attempts have been made [1]}, [2]}, [3]}, [4]}, [5]}, [6]} to derive a completely positive QME in Lindblad form from the Redfield equation without resorting to secular approximation. These studies demonstrate that there is no specific way to derive a QME in Lindblad form: as noted in Ref. [1]}, there is a continuous family of Markovian QMEs within the same order of precision. This naturally raises the question of which QME is most appropriate to describe a given open quantum system, and the physical basis on which a given QME can be justified.
